1. Field of the Invention
The present invention relates to a pixel and an organic light emitting display device using the same.
2. Discussion of Related Art
In recent years, there have been may attempts to develop various flat panel display devices with reduced weight and volume, as compared to cathode ray tubes. Flat panel display devices include liquid crystal display devices, field emission display devices, plasma display devices, and organic light emitting display devices, among others.
Among flat panel display devices, the organic light emitting display device displays an image by using organic light emitting diodes, which generate light by recombining electrons and holes. The organic light emitting display device has an advantage in that it has a relatively rapid response time and may also be driven with relatively low power consumption.
FIG. 1 is a circuit view showing a pixel of a conventional organic light emitting display device.
Referring to FIG. 1, the pixel 4 of the conventional organic light emitting display device includes an organic light emitting diode (OLED), and a pixel circuit 2 coupled to a data line (Dm) and a scan line (Sn) to control the organic light emitting diode (OLED).
An anode electrode of the organic light emitting diode (OLED) is coupled to the pixel circuit 2, and a cathode electrode of the organic light emitting diode (OLED) is coupled to a second power source (ELVSS). The organic light emitting diode (OLED) generates light with a luminance corresponding to an electric current supplied from the pixel circuit 2.
The pixel circuit 2 controls an amount of current supplied to the organic light emitting diode (OLED) in accordance with a data signal supplied to the data line (Dm) when a scan signal is supplied to the scan line (Sn). For this purpose, the pixel circuit 2 includes a second transistor (M2′) coupled between a first power source (ELVDD) and the organic light emitting diode (OLED); a first transistor (M1′) coupled between a gate electrode of the second transistor (M2′) and the data line (Dm), with a gate electrode of the first transistor coupled to the scan line (Sn); and a storage capacitor (Cst) coupled between the gate electrode and a first electrode of the second transistor (M2′).
The gate electrode of the first transistor (M1′) is coupled to the scan line (Sn), and a first electrode of the first transistor (M1′) is coupled to the data line (Dm). A second electrode of the first transistor (M1′) is coupled to one terminal of the storage capacitor (Cst). Here, the first electrode of the first transistor (M1′) is either a source electrode or a drain electrode, and the second electrode of the first transistor (M1′) is the other of the source electrode and the drain electrode. For example, when the first electrode is a source electrode, the second electrode is a drain electrode. The first transistor (M1′) is turned on when a low scan signal is supplied from the scan line (Sn), and supplies a data signal from the data line (Dm) to the storage capacitor (Cst). In this case, the storage capacitor (Cst) is charged with a voltage corresponding to the data signal.
The gate electrode of the second transistor (M2′) is coupled to one terminal of the storage capacitor (Cst), and a first electrode of the second transistor (M2′) is coupled to the other terminal of the storage capacitor (Cst) and the first power source (ELVDD). A second electrode of the second transistor (M2′) is coupled to an anode electrode of the organic light emitting diode (OLED). The second transistor (M2′) controls the amount of current in accordance with a voltage value stored in the storage capacitor (Cst), the current flowing from the first power source (ELVDD) to the second power source (ELVSS) via the organic light emitting diode (OLED). In this case, the organic light emitting diode (OLED) generates light corresponding to the amount of current supplied from the second transistor (M2′).
However, the pixel 4 of the conventional organic light emitting display device has problems with displaying an image with uniform luminance. More particularly, a threshold voltage of the second transistor (M2′) (e.g., driving transistor) in each of the pixels 4 is different due to manufacturing process variances. When the threshold voltages of drive transistors are set to different threshold voltage levels as described above, inaccurate luminance is generated in pixels of the organic light emitting diode (OLED) due to the different threshold voltages of the drive transistors, even though data signals corresponding to a same gray level are supplied to the pixels.
Also, the conventional organic light emitting display device has problems in that the voltage from the first power source (ELVDD) may vary from pixel to pixel due to a voltage drop of the voltage from the first power source (ELVDD), depending on the position of each pixel in the display unit. When the voltage from the first power source (ELVDD) varies according to the position of the pixels as described above, it is very difficult to display an image with uniform luminance.
Furthermore, the conventional organic light emitting display device has problems displaying images with desired luminance due to the changes in efficiency from degradation of the organic light emitting diode (OLED). That is to say, organic light emitting diodes (OLED) degrade with time, which makes it more difficult to display an image with desired luminance. In fact, the organic light emitting diode (OLED) device generates images with progressively lower luminance as the organic light emitting diodes (OLED) degrade.